The amount of nitrogen oxides (hereinafter referred to as NOx) which forms on combustion of a gaseous, liquid or solid fuel depends on combustion conditions, especially such factors as the flame temperature, oxygen concentration and residence time of burnt gases in the high temperature region, in which the higher the flame temperature and the higher the oxygen concentration, the larger the amount of NOx is produced.
In conventional methods, a burner tip end is kept away from an inner furnace end surface as far as possible, provided that carbon deposits do not accumulate on the surface of the burner tile, not only to increase combustion efficiency but also to shorten the flame length whereby the fuel is quickly mixed with the combustion air and it is ensured to obtain a quick combustion similar to premixture combustion starting from the burner tile bore.
The residence time of burnt gases in the high temperature region is shortened due to the quick mixing and combustion because of the short flame which results in a small radiant heat loss from the flame. Such a quick mixing and combustion elevates the flame temperature and increases the localized oxygen concentration in the combustion zone, with the consequent result of the formation of a large amount of NOx.